The former couple, who divorced in 2011 after three years of marriage, met up in the Studio City neighbourhood of Los Angeles on Saturday (16.11.13).
The 29-year-old star and 'Fall Out Boy' singer each brought their new significant others, Meagan Camper and Evan Ross, to the family gathering held at The Coop, a party space for children.
A source told Us Weekly: "It didn't seem awkward at all between Evan Ross and Pete Wentz. They took a family photo and were all in it -- Pete, Meagan, Ashlee and Evan."
The source said 34-year-old Pete was first to arrive with the birthday boy with his girlfriend of two years, Meagan, and his parents in tow.
The rocker was photographed holding hands with his new love, and shortly after Ashlee arrived with her boyfriend Evan, 25, and his mother, the legendary singer Diana Ross, along with her own mother Tina Simpson.
The friendly exes threw their son a Disney-themed party with balloons featuring famous characters from 'Cars', 'Monsters Inc.' and 'The Incredibles'.
There were reportedly 75 adults and 45 kids in attendance at the party and Ashlee's sister Jessica, who has daughter Maxwell, 18 months, and son Ace, four-months, with fiancé Eric Johnson, were photographed talking with other guests outside the venue.